Today, public telephones are easily accessible, but often located in places that are very noisy (e.g., streets, restaurants, train stations, airports, etc . . . ). Given these circumstances, voice communications (e.g., telephone conversations) sometimes become unpleasant and stressful. A noisy environment severely reduces the “intelligibility” (i.e., clarity or understanding) of the words being spoken or heard. The rising popularity of cellular phones, which are also used in noisy environments, increases the need to develop an adequate solution for this problem.
Intelligibility losses due to background noise (sometimes referred to as “ambient” noise) are well known. One solution to reduce the impact of background noise on intelligibility uses a “clipping” technique (see I. B. Thomas, R. J. Niederjohn, “Enhancement of speech intelligibility at high noise levels by filtering and clipping,” J. of the Acoust. Soc. of Am., Vol. 16, 1968, pp. 412–415). Although clipping improves intelligibility, it adds distortion to the signal. Alternatively, others have attempted to improve intelligibility using limiters (see E. A. Kretsinger, N. B. Young, “The use of fast limiting to improve the intelligibility of speech in noise,” Speech Monogr., vol. 27, 1960, pp. 63–69), high-pass filters, dynamic compression, or some combination of these R. J. Niederjohn, J. H. Grotelueschen, “The Enhancement of Speech Intelligibility in High Noise Levels by High-Pass Filtering Followed by Rapid Amplitude Compression,” IEEE Trans. on Acoustics, Speech and Signal Proc., Vol. ASSP-24, No. 4, August 1976, pp. 277–282).
Telephone manufacturers have placed volume controls (e.g., on telephone handsets) in an attempt to solve background noise problems. However, these controls are inconvenient and often ineffective, particularly when they are used in an attempt to compensate for rapidly changing background noise.
Alternatively, automatic compensation techniques have been developed. The process of automatically compensating for background noise—referred to as “noise compensation”—provides significant benefits. Such techniques respond faster to a changing environment. Simple noise compensation methods, also referred to as noise-adaptive automatic level controls, have been used by automotive radio manufacturers for audio reproduction in background noise (see U.S. Pat. No. 4,628,526, “Method And System For Matching The Sound Output Of a Loudspeaker To The Ambient Noise Level” H. Germer), as well as cellular phone manufacturers (see U.S. Pat. No. 5,509,081, “Sound Reproduction System,” J. Kuusama). However, these simple automatic level controls do not reduce the dynamic range of an audio signal. Therefore, soft signal portions may get lost among the background noise while loud portions may be too loud for a listener. These effects reduce the overall benefit of such techniques.
Other techniques address the dynamic range problem by incorporating a dynamic compressor. Compressors have been used by audio and telephone manufacturers (see U.S. Pat. No. 5,107,539, “Automatic Sound Volume Controller, Kato, et. al; and E. F. Stikvoort, “Digital dynamic range compressor for audio,” J. Audio Eng. Soc., Vol. 34, No. 1/2, January/February 1986, pp. 3–9).
In telephony applications, noise compensation techniques involve automatically compensating for “near-end” (i.e., the location under consideration) background noise by enhancing or “amplifying” a “far-end” (i.e., the location of the other end) signal. Existing compressors have been suggested for applications in both telephone sets (see U.S. Pat. No. 5,553,134, “Background Noise Compensation In a Telephone Set;” J. B. Allen, D. J. Youtkus) and networks (see U.S. Pat. No. 5,524,148, “Background Noise Compensation In a Telephone Network,” J. B. Allen, D. J. Youtkus). Such compressors have their limitations, however. Existing compressors are generic versions of audio compressors. Generic audio compressors do not adapt their characteristics to an external input, such as a noise level.
For example, circumstances arise where the level of noise changes from a relatively low level to a relatively high level. Unfortunately, existing compressors do not adapt their operating characteristics in accordance with such changes. This means that sometimes too much or too little compensation is applied to a signal.
Some existing techniques rely solely on the detection of near-end noise levels, failing to account for far-end noise. Such techniques wind up amplifying not only the desired signal but also the noise level contained in such a signal as well. The result is that a desired signal and an undesired signal (e.g., noise) are amplified by the same amount.
Another consideration, related to the “sensitivity” of a handset's microphone (i.e., the output of a microphone at a given sound pressure level), is also commonly overlooked by existing noise compensation techniques. A microphone in a handset picks up speech and background noise. The sensitivity of the microphone affects the estimate of the noise level. Because existing techniques fail to account for the sensitivity of a microphone they cannot provide an accurate amount of compensation. Instead, existing systems provide a level of compensation which may be too low or too high to correctly compensate for noise initially received by the microphone. Typically, existing compressors include a device known as a “noise adaptive gain” controller (“NGC”) which is used to provide compensation based on an assumed average sensitivity. If an NGC is providing an incorrect amount of compensation, this error will also cause other parts of the compressor to provide an incorrect amount as well.
When noise compensation techniques are implemented in a network, the problem of “unknown network gain” is added to the problem of inaccurate knowledge of a microphone's sensitivity. For example, a near-end signal may be amplified or attenuated (e.g., by an automatic level control device) before arriving at a location in the network where noise compensation is being carried out. As a result, the electric signal level can no longer be used to derive the sound pressure level at the handset. Existing techniques fail to recognize this problem and, as a result, derive noise level estimates that are often heavily biased which results in too little or too much noise compensation.
Accordingly, more effective noise compensation methods and systems are desirable for increasing the clarity/intelligibility of voice communications.
Other desires will become apparent from the drawings, detailed description of the invention and claims that follow.